The invention is concerned with flat screen displays, and in particular the invention relates to a flat, thin cathode ray tube structure which utilizes a flat, generally uniform array of electrons, passed selectively through an addressing grid to address pixels on an electron-excitable coated face plate. This is in contrast to a scanned ray of electrons as in conventional CRTs.
Flat screen video displays have been known in concept and have been a common goal in the video/television industry for some years. For examples of this and related technology, see U.S. Pat. Nos. 3,566,187, 3,612,944, 3,622,828, 3,956,667, 4,088,920, 4,227,117, 4,341,980, 4,435,672, 4,531,122, 4,564,790 and 4,719,388. Such flat screen display structures have been intended to eliminate the very deep profile of televisions and other CRT displays, required because of the electron scanning gun which must be a certain proportionate distance behind the phosphor coated face plate, this distance increasing with screen size. Other goals of flat screen television have been reduction of weight, avoiding the requirement of high voltage for larger screens, truly flat face plates, and reduced cost of manufacture.
Numerous alternative thin, flat screen technologies have been developed which are either currently used in display applications or show distinct promise for such applications. These applications typically require low power, require light weight and/or small size (characteristics provided in varying degrees by these flat screen displays) and do not require the video speeds, full color, high screen resolution, or other features that can currently only be accomplished by conventional cathode ray tube displays. Hence, although many new applications have developed for flat screen technologies, these technologies have not significantly penetrated the existing large applications for CRTs, such as television and desktop computer.
For example: the conventional twisted pneumatic and supertwist liquid crystal displays ("LCDs") have very low power and cost in monochrome modes, but do not have sufficient speed, gray shades, uniformity, power efficiency and resolution to be used in television and many computer applications, which require full color and video rates. More advanced LCD technologies have also been produced, such as ferroelectric LCDs which switch at video speeds. However, this technology still has significant gray scale, manufacturing, reliability and life problems which must be overcome before it can be used in full color and video display applications.
Another advanced LCD technology, active matrix LCDS, uses thin film transistors or diodes at the location of each picture element to switch the liquid crystal material at video speeds and achieve very high resolutions without losing contrast. Although this technology has the potential to be used in full color and video display applications, it is very difficult (i.e. expensive) to manufacture. The thin film circuits at each pixel are a few microns in size, and must be made with submicron registrations between the thin film layers over 8" or more of a glass substrate. This glass substrate expands and contracts nonuniformly during manufacturing processing (among other significant manufacturing problems).
Thin film electroluminescent, TFEL, displays can display information at video rates and potentially can be made in large sizes. However, they are relatively power inefficient because electron to light conversion efficiencies are very low for color TFEL phosphors and because the capacitive loading from the thin film electrode/dielectric structures is relatively high. TFEL displays are expensive to produce because the higher voltage and current requirements necessitate expensive drive circuits, and also because the thin films must be pinhole free over the entire display area to avoid shorts in addressing electrodes.
Plasma and vacuum fluorescent displays have been produced with video rate speeds and color, but these also remain relatively power inefficient and expensive due to manufacturing difficulties and expensive drive electronics.
One of the most promising approaches for duplicating the full color, wide viewing angle, high power efficiencies, high resolution, large areas and high brightness characteristics of the CRT is to develop a "flat CRT". Numerous flat CRT technologies have been developed to various degrees, but all have been too expensive for TV and other large volume display applications and have not been scalable to large sizes. One flat CRT approach uses conventional electron beam scanning, but magnetically folds or bends the electron beam so that the resulting tube can be relatively thin. This approach has worked for small displays, but suffers significant image distortion and resolution loss when scaled up to sizes much beyond three inch diagonal screens.
Another flat CRT approach involves the use of micron sized field emitters which can emit electrons into vacuum without the heating required in conventional thermionic cathodes. This has the potential of providing a very efficient and thin flat CRT. However, addressing of the individual emitters is difficult.
Also, there remain other significant manufacturing, reliability and uniformity problems yet to be resolved before flat CRTs using field emission cathodes can be manufactured in volume and at realistic cost.
Most of the remaining flat CRT approaches have used one or multiple grid structures to switch on and off a matrix of micro electron beams. These beams originate ideally from a planar source of electrons (emitted from a distributed set of cathodes) on one side of the grid structure and are accelerated forward on the other side of the grid toward phosphors on an anode plate, which is maintained at a high voltage potential. Although prototype monochrome and color displays using this general approach have been produced, the fabrication costs, assembly difficulties and/or performance characteristics of the grids were major reasons why each such prototype failed to meet its commercial cost and performance targets.
A number of the above-listed patents were concerned with flat screen cathode ray tubes. These have generally not functioned as desired and sometimes not precisely as explained theoretically in the patents, and many of them have been too costly to achieve with the required level of reliability. Yields can be very low. Some of the patented arrangements were based on incorrect premises as to behavior of electrons presented behind an addressing grid in a supposed uniform planar array for availability in exciting pixels with desired resolution and at prescribed brightness. No flat screen cathode ray tube described in the referenced patents has yet reached commercial significance.
The addressing grid structure is a problem for which adequate solutions have not yet been presented by any of the flat screen CRTs described in the prior art. In order for an addressing structure to efficiently and reliably address individual pixels, without any dead spots on the entire screen, there must be an efficient means of placing the appropriate positive electrical charge at individual addressing points so as to accelerate electrons toward the prescribed pixels, without an inordinate volume of wiring or a complex maze of conductive traces or printed circuit patterns. Proposals for high definition television have included displays with up to 1152 rows and with up to 2048 columns, i.e. more than two million pixels in the display. A 14-inch diagonal HDTV display (following a prevailing view of a 9:16 aspect ratio) is 6.86 inches high. The color triads in such a display are only 6.0 mils apart. This presents real problems as to the dimensions of any wire grid addressing structure. If wires were to be used, extending in free space, they would have to be of sufficiently small dimension to leave adequately sized openings such that an appreciable number of electrons can be admitted through the grid to the pixels, as compared to the number of electrons which will flow into the grid itself as current.
Another consideration in the design of a flat screen CRT video display is support for the glass, phosphor coated (anode) face plate under the near-perfect vacuum which must exist inside the CRT. Thick, arched glass must be avoided. Previously suggested flat screen CRT structures simply have not addressed this issue with any practical and cost-efficient structure. At the rear of the flat screen CRT, the same issue exists with respect to the support of a rear plate which closes the back of the display.
Further considerations with the development of an efficient, dependable, cost effective flat screen CRT having adequate brightness capability and reasonable longevity include producing a reliable source of electrons, uniformly distributed, for use in addressing the display tube pixels; and sealing a flat screen structure reliably to retain the high vacuum while bringing a multiplicity of conductive paths to the exterior of the flat picture tube for inputting the addressing signal to the addressing grid and for other purposes. "Hot" or thermionic cathodes have been the usual suggested means for achieving the desired electron cloud, as in U.S. Pat. Nos. 4,719,388, 4,435,672 and 3,566,187 referenced above. "Cold" cathodes have been suggested in various configurations but as yet have not proven to be cost-efficient, effective and reliable for use in repeatedly addressing the very large number of pixels in a video display, particularly in a high definition display. Examples of attempts at a cold cathode are the efforts of LETI (France) and Coloray Corporation of Fremont, Calif. to achieve a cold cathode for a flat screen display using microtip technology. One problem has been that the microtips are not sufficiently uniform from tip to tip to achieve predictable pixel activation if each tip is relied upon. Thus, a group of hundreds of microtips has been used to supply electrons for one pixel dot on the screen. The approach attempts to apply integrated circuit technology to full screen dimensions, requiring wiring to active transistors over a large area and leading to other problems as well. Further, ion milling problems from backflow of ions require the use of low voltage phosphors, which are of lower efficiency than high voltage phosphors and cannot be aluminized, thus further reducing their efficiency due to loss of rearward directed photons.
A disclosure of an addressing grid structure was published by Northrop Corporation in 1974, entitled "Digital Address Thin Display Tube", by Walter F. Goede, distributed by National Technical Information Service (U. S. Department of Commerce). The disclosure describes a fritted together stack of glass plates having conductive strips deposited on the glass plates and a multiplicity of holes through the plates. Low temperature glass was used, so that the plates could be fused together in the fritting process at relatively low temperature. However, the Northrop disclosure involves pure amorphous glass plates, assembled in a rigid state, rather than unfired ceramic or glass/ceramic layers or other initially flexible sheet material. The amorphous glass plates are weak as compared to glass ceramic plates. Further, the fritted stack of glass plates was to be placed inside a vacuum tube, rather than sealing directly to the stack of plates and rather than being self-supporting against the face plate as in the present invention. The thick addressing grid would make electron transmission difficult and efficiency low. Further, hole forming/trace printing sequences as explained below relative to the present invention could not have been used, and the density of holes was limited. The Northrop structure differed markedly from the present invention in these and other respects.
See also "A Digitally Addressed Flat-Panel CRT", by W. F. Goede, EEE Transactions on Electron Devices, Vol. Ed-20, No. 11, November 1973, disclosing a multiple-plate addressing structure and encoding techniques.
Other work with flat-screen CRT displays has been done by Texas Instruments and by Source Technology. The Texas Instruments' work involved a grid of conductive ribbons formed by a photo etching process, each ribbon coated by a glass frit. The ribbons were overlaid in a grid and the assembly was heated to fuse the glass coated ribbons together. This produced a very fragile grid assembly, and one wherein shorts would frequently occur at the conductive ribbon crossovers due to nonuniformity in the glass layers. Yields were extremely low, so low as to not be economical in manufacture.
The work done by Source Technology involved leads on a substrate having only a top and a bottom surface. The hermetic seal for the assembly was made directly over the leads. Source Technology's substrate was a Photoceram (a trademark of Corning) sheet, with etched holes and deposited conductive traces formed by a solid sheet of conductor which was then divided by lasercutting. Addressing density of the grid holes was not adequate for most of today's applications.
The following table lists features of different previous approaches as outlined above, in comparison with the system of the present invention.
TABLE I ______________________________________ PRESENT INVEN- A B C TION ______________________________________ Grid Substrate: Flexibility dur- moder- poor poor excellent ing Initial ate Manufacturing Handleability poor excellent moderate excellent after Fabricat. Integral Supports no no no yes for Enclosure Possible Ease of Hole poor poor excellent excellent Formation Ease of Trace poor excellent poor excellent Formation Material Compatilibity: Cathode good good poor good Poisoning Vacuum poor moderate excellent excellent outgassing Functionality: Hermetic Leadout no no yes yes Integral with Grid Encoding Possible yes yes no yes Ability to Mount no no moderate excellent chips Directly on Grid ______________________________________ A: Glass frit coated conductive ribbons B: Fritted glass plates C: Single sheet photochemically active glass
Previously described flat screen displays failed to provide or suggest an efficient, manufacturable, high-yield, cost-effective and reliable system for a flat screen video display tube, as in the present invention described below.